According to Karwarth in the Journal of the Institute of Navigation, Vol. 24, No. 1, pp. 105-120, Jan. 1, 1971, the basic objective of area navigation is to position in real time with appropriate accuracy one or more mobile units with reference to some known coordinate system. The number of coordinates needed depends upon whether the course of the mobile unit can be charted on a known surface or must be described in three-dimensional space as in the respective cases of a ship at sea and an aircraft. The ability to chart a course based on past, present and future desired positions is a principal element in distinguishing an area navigation system from navigation using point-to-point or "homing" approaches such as VOR/DME (VHF Omnidirectional Range/Distance Measuring Equipment).
Positions are established in all cases by signal transmission between the mobile units and at least one transmitter of known location. The transmissions can be electromagnetic (including optical) or acoustic in any medium including air. Two basic methods are normally used to obtain positions:
Positions may be determined from a sufficient number of range measurements to known reference locations by using what is commonly known as "range-range" systems, or positions may be determined from a sufficient number of range differences to known reference locations, by using what is commonly known as "hyperbolic" systems. In each case a sufficient number is at least equal to the number of coordinate values needed.
Direct ranging involves calculation of intersections of the circles or spheres of uniform range from each reference location to the mobile unit. By contrast, the locus of equal range difference from the mobile unit to a pair of reference locations are hyperbolae or hyperbolae of revolution. Again, positions are calculated by intersection of curves or surfaces, but in this case related to hyperbolae, hence the name hyperbolic systems.
An exemplary task of area navigation might be the positioning of a ship at near shore distances. In range-range operation only two shore stations are needed while a hyperbolic system requires three. Three shore stations admit calculation of three range differences hence providing some element of data redundancy. As a general rule, hyperbolic systems offset a disadvantage in requiring one more known reference location than the simplest operable ranging system by having some degree of data redundancy.
As for the transmitted signals, a variety of differing modes of operation are possible. Ranges can be determined from signal transmission time between the mobile unit and a reference location if the signal initiation time is known, a time standard is available, and a signal propagation velocity is also known. The simplest means for establishing known initiation times is to transmit signals only in response to some interrogation. Alternatively, if transmissions are synchronized to occur at regular time intervals, time differences are readily determined with no interrogation step needed using only a local time standard and a propagation velocity.
Again, signal transmissions themselves can consist of continuous waveforms (typically sinusoids), intervals of continuous signal transmissions, or sequences of pulses. The choice of transmitted signal reflects consideration of the information desired, mode of operation (range-range versus hyperbolic), noise effects, and the extent of Doppler distortions amongst other factors.
Continuous waveforms are the most robust signals in the presence of noise backgrounds since correlation-type receivers may take advantage of the extreme signal duration. Such signals have no resolution in time and are used principally with hyperbolic systems to establish time-differences by making phase comparisons with reference signals. Where the transmitter-receiver relative velocity is not insignificant in proportion to the signal propagation velocity, Doppler effects shift the frequencies of continuous waveforms. Frequency shifts may be viewed as errors since they distort subsequent correlation steps and thus degrade phase comparisons, however, if such shifts are measured, they do relate to velocity information should this be desired.
Use of continuous signals over intervals provides time resolution as well as opportunities for correlation detection, but over shorter data windows. Again, any Doppler effects may be viewed either as constituting an error in range determination or if measured, velocity information. The tolerance of such signals to noise effects is of course diminished in direct proportion to their shortened duration.
In the limit, as duration is shortened, pulsed signals must be considered which when taken individually offer no opportunity to measure Doppler effects. Hence significant transmitter-receiver relative velocities will be noted as range errors for such systems. These signals are also most affected by the presence of noise, but afford the greatest resolution in making a direct time measurement.
It follows that the alternative methods of operations which exist constitute attempts at optimizing a number of trade-offs which interact with some complexity. The hardware requirements, operation costs, efficiencies and effectiveness in terms of achievable accuracy are all essential ingredients which play roles in the optimization.
The following Table presents a sampling of a number of commercially available area navigation systems. A much more detailed and comprehensive tabulation of short and medium range electromagnetic position fixing systems was presented by Rear Admiral C. Munson at the XV Annual Congress of Surveyors, Stockholm, June 1977. The variety of candidate systems gives insight into the way in which the optimization problem has been addressed. Navigation systems based on the present invention may serve as replacements for each of the systems noted in the following table amongst others.
TABLE ______________________________________ OF REPRESENTATIVE COMMERCIAL AREA NAVIGATION SYSTEMS Operating System and Mode and Range Company Model Environment Frequency (nm.) ______________________________________ Alpine SUNS 3 transponders- 11 khz. trans- 6 Geo- (Sonic underwater ducer 13, 14, physical Under- ranging 15 khz. trans- Assoc. Inc. water ponders. Norwood, Navigation N.J. system) Model 775 Cubic Cor- Autotape Interrogator 2900-3100 93 poration Model and 2 re- mhz. San Diego, DM-40 sponders - Ca. (Electronic surface ranging Positioning system) Decca Pulse/8 Receiver and 3 100 khz. 300+ Survey or more trans- Systems mitters - hyper- Inc. bolic or ranging Houston, Tx. Long Receiver and 2 300-400 mhz. 200+ Range or more trans- Shoran mitters - surface ranging Hi-Fix/6 Receiver and 2 1.6-5.0 mhz. 100+ or more trans- mitters - hyper- bolic or ranging Sea-Fix Receiver and 2 2 mhz. &lt;100 or more trans- mitters - hyper- bolic or ranging Trisponder Mobile station Microwave &lt;50 and at least two (X Band). remotes - 9350 mhz. surface ranging. (mobile) 9450 mhz. (remote) Aquafix Surface trans- 10.5-16 khz. &lt;1 mitter with bottom trans- ponders or hydrophone with radio link - slant ranging del Norte Trisponder Mobile master Microwave &lt;50 Tech- Model 202a and two slaves - (X Band) with nology, surface ranging 9450 mhz exten- Inc. sion to Euless, Tx. &lt;150 Motorola, RPS Receiver/Trans- Microwave 50 Scottsdale, (Range mitter and two (X Band) exten- Arizona Positioning or more Radar 9300-9500 sion to System) transponders - mhz. 100 surface ranging Mini- Receiver/Trans- Microwave &lt;20 Ranger mitter and two (X Band) exten- III System or more Radar 5450-5600 sion to transponders - mhz. &lt;100 surface ranging Ocean Transnav Transducer and 7.5 (on approx. Research 6000 four trans- board) or 12 Equipment, Acoustic ponders - 8.57 (sub- Inc. Navigation surface and marine)khz Falmouth, System under-water transducer Mass. ranging 10.75, 11.25, 12.25 khz transponders Teledyne Raydist-76 Mobile station 1600-4000 khz approx. Hastings- System and and two or 200 Raydist, DRS-H three shore sta- day- Hampton, System tions - ranging time Va. and hyperbolic approx. (for 3rd station) 120 night- time. ______________________________________